The manufacture of semiconductor devices such as integrated circuits typically requires heat treating silicon wafers in the presence of reactive gases. During this process, the temperatures and gas concentrations to which the devices are exposed must be carefully controlled, as the devices often include circuitry elements less than 1 um in size which are sensitive to minute variations in the processing environment.
The semiconductor manufacturing industry typically processes these wafers in either horizontal or vertical carriers. The horizontal carrier, typically called a "boat", has three or four horizontally disposed bars arranged in a semicircle design, with each bar having inwardly facing grooves set therein at regular intervals. Each set of grooves define a vertical space for carrying a vertically disposed wafer. The vertical carrier, typically called a "vertical rack", has three or four vertically disposed rods (or posts) arranged in a semicircle design, with each post having slots set therein at regular intervals to define a space for supporting a horizontally disposed wafer. To insure the geometrical precision required in this field, the three or four rods are fixed to a top plate and a bottom plate. The portions of the post between each slot, termed "teeth", are identically spaced in order to support wafers at regular intervals from and parallel to the bottom plate. The entire rack is then placed within a vertical furnace for processing the wafers.
Because a wafer processed on a vertical rack experiences less of a temperature gradient over its face (as compared to a wafer processed in a horizontal boat), semiconductor manufacturers are increasingly turning to vertical furnaces. There is, however, a drawback to vertical furnacing. The wafers disposed on a conventional vertical rack are supported at their outside edge only. As such, the areas of the wafer resting on these teeth experience higher stress than the rest of the wafer.
When temperatures in the furnace exceed about 1000.degree. C., these stresses often become significant and portions of the single crystal wafer move relative to each other along crystallographic plates in response to that stress. This phenomenon, called "slip", effectively destroys the value of the semiconductor devices located in the area of the wafer where slip has occurred.
In response to the slip problem, the art has developed a host of vertical racks having extended, vertically-inclined teeth, whereby a horizontally-disposed wafer supported thereon is supported only in a region near its center. See e.g., U.S. Pat. No. 5,492,229 ("Tanaka"), FIG. 2b of WO 96/35228 ("Tomanovich") and FIGS. 5-7 of U.S. Pat. No. 5,586,880 ("Ohsawa"). These configurations provide what the art has termed "near-center support". Although the rack which produces "near center support" has eliminated slip in many wafer processing applications, it is somewhat inefficient, as the portion of its arm which rises from the horizontal plane to produce the near-center support necessarily increases the vertical spacing period of the rack, that is, the wafer-to-wafer distance. Tanaka, for example, specifically teaches that its riser vertically extend at least about 0.3 mm from the flat portion of the tooth. Accordingly, the near-center support style of vertical rack can typically accommodate only about 90% of the wafers typically accommodated by the conventional vertical rack.
FIG. 3(b)of Tomanovich discloses a vertical rack wherein the arms are strictly horizontal and do not rise. Although this rack has been found to eliminate slip in some high temperature semiconductor wafer processing applications, it performs less well in more demanding applications.
Therefore, there is a need for a vertical rack which is as efficient as the conventional style rack and yet eliminates slip in more demanding wafer processing applications.